Infiltration of material with silicon

ABSTRACT

A process for producing a composite by embedding a porous body at least partly composed of a substance which reacts with silicon in a powder mixture composed of silicon and hexagonal boron nitride powder, heating the resulting structure to a temperature at which the silicon is fluid and infiltrating the fluid silicon into the porous body.

The present invention relates to a process for infiltrating a porousbody of a material with silicon.

The infiltration of materials with silicon has previously been carriedout by using some form of a carbon fiber wick to transport liquidsilicon from a reservoir to the material being infiltrated. U.S. Pat.Nos. 4,120,731; 4,141,948; 4,148,894; 4,220,455; 4,238,433; 4,240,835;4,242,106; 4,247,304 and 4,353,953, assigned to the assignee hereof andincorporated herein by reference disclose such infiltration of siliconusing a carbon wick. After infiltration, this wick becomes an integralpart of the infiltrated body and requires removal by diamond machining.For very small or thin bodies, not only is removing the wick a problem,but in addition, the molds necessary for the silicon infiltration ofsome shapes are rather complex.

According to the present invention, materials can be infiltrated withsilicon by embedding them in a mixture of silicon and hexagonal boronnitride powders. The vacuum pressure, furnace temperature, and time attemperature are the same, or substantially the same, as used in thetechniques employing carbon wicks.

Briefly stated, in one embodiment, the present process for infiltratinga porous body of a material with silicon to form a composite comprisesproviding a material wherein at least about 5% by volume of saidmaterial comprises a component which reacts with silicon, said materialhaving a melting point higher than that of said silicon, forming aporous body of said material having an open porosity ranging fromgreater than about 10% by volume to about 90% by volume of said body,contacting said body with a powder mixture comprised of a powder of saidsilicon and hexagonal boron nitride powder wherein said silicon powderranges in amount from greater than about 10% by volume to less thanabout 90% by volume of said mixture, heating the resulting structure ina nonoxidizing partial vacuum to a temperature at which said silicon isfluid but below the melting point of said material and which does nothave a significantly deleterious effect on said material andinfiltrating said fluid silicon into said porous body forming saidcomposite, said partial vacuum being at least sufficient to remove gasfrom said porous body which blocks said infiltrating fluid silicon.

Briefly stated, in another embodiment, the present process forinfiltrating a porous body of a material with silicon to form acomposite comprises forming a mixture comprised of a powder of saidsilicon and hexagonal boron nitride powder wherein said silicon powderranges in amount from greater than about 10% by volume to less thanabout 90% by volume of said mixture, shaping said mixture into a moldhaving a cavity of the size and shape of said composite, providing amaterial wherein at least about 5% by volume of said material comprisesa component which reacts with silicon, said material being in the formof particles, filaments, cloth or mixtures thereof and having a meltingpoint higher than that of said silicon, packing said material into saidcavity producing a packed material or porous body therein having an openporosity ranging from greater than about 10% by volume to about 90% byvolume of said packed material or porous body, heating the resultingstructure in a nonoxidizing partial vacuum to a temperature at whichsaid silicon is fluid but below the melting point of said material andwhich does not have a significantly deleterious effect on said materialand infiltrating said fluid silicon into said packed material or porousbody forming said composite, said partial vacuum being at leastsufficient to remove gas from said porous body which blocks saidinfiltrating fluid silicon,

In carrying out the present process, a mixture of elemental siliconpowder and hexagonal boron nitride powder is formed and contacted withthe surface of the porous body or packed material to be infiltrated.Since molten silicon does not wet or react with the hexagonal boronnitride powder, at the temperatures used, it does not coalesce, andtherefore, permits easy movement by vapor and liquid flow to thesurfaces of the porous body or constrained material where it reacts,wets and infiltrates. Without the hexagonal boron nitride powder therewould be no control over the silicon infiltration. Specifically, thesilicon would not infiltrate the body uniformly and would form dropletson its surface thereby leaving significantly large silicon nodulesthereon which would require diamond machining for their removal.

The silicon powder can range widely in size but preferably should not begreater than about 100 mesh, i.e. no greater than about 150 microns,since a larger particle size would have a tendency to coalesce and notinfiltrate the body. Preferably, the silicon powder has a particle sizeof about 200 mesh, i.e. no greater than about 75 microns.

The hexagonal boron nitride powder can range widely in size butpreferably should not be greater than about 100 mesh, i.e. no greaterthan about 150 microns, since a larger particle size may allow thesilicon to coalesce thereby preventing its infiltration into the body.Preferably, the hexagonal boron nitride powder has a particle size ofabout 325 mesh, i.e. no greater than about 45 microns.

As used herein by mesh it is meant U.S. Sieve Size.

In the present powder mixture of silicon and hexagonal boron nitride,the silicon powder ranges in amount from greater than about 10% byvolume to about 90% by volume of the mixture depending largely on therate at which the infiltration of the fluid silicon into the porous bodyis to be carried out. The smaller the amount of silicon in the mixture,the slower will be the rate of infiltration. Preferably, the siliconcontent of the mixture ranges from about 50% by volume to about 85% byvolume, more preferably from about 60% by volume to about 80% by volume,of the mixture. Silicon should be present in the powder mixture at leastin an amount sufficient to produce the desired composite.

The mixture of silicon and hexagonal boron nitride powders can be formedby a number of conventional techniques. For example, the two powders cansimply be mixed together to produce the present mixture. At least asignificantly uniform mixture of the two powders is formed, andpreferably a uniform or substantially uniform mixture is formed.

The present mixture of silicon and hexagonal boron nitride powders canbe used in a variety of forms, i.e. it can be placed in contact with thematerial to be infiltrated by a number of techniques. For example, thepowder mixture can be in the form of a pressed powder or in the form ofa mold with a cavity of the size and shape desired of the finalinfiltrated body or composite. Before infiltration, when in contact withthe porous body or compacted material, the present mixture preferablyhas a porosity of less than about 50% by volume, more preferably lessthan about 40% by volume, and still more preferably less than about 30%by volume of the powder mixture.

In one embodiment, supporting means, preferably graphite or otherelemental carbon, provided with a cavity are used and the present porousbody is embedded in the present powder mixture in the cavity.Preferably, the resulting assembly is heated to infiltrationtemperature. Any supporting means used in the present process shouldhave no significant deleterious effect thereon. Also, preferably, thecavity of any supporting means is precoated with a parting agent such ashexagonal boron nitride to prevent sticking.

The present mixture of silicon and hexagonal boron nitride powders canbe formed into a mold with the desired cavity by a number of techniques.In one embodiment, cavity-containing supporting means, preferablygraphite or other elemental carbon with a cavity machined therein, isused and a layer of the mixture is pressed against the inner surface ofsuch cavity producing the desired mold. In another embodiment, themixture can be extruded, injection molded or die-pressed to produce amold with a cavity of the size and shape of the composite. Anylubricants, binders or similar materials used in shaping the powdermixture should have no significant deleterious effect in the presentprocess. Such materials are of the type which evaporate or decompose onheating below the present infiltration temperature, preferably below500° C., leaving no deleterious residue. Representative of usefulbinders are liquid epoxy resin and water. In one embodiment a liquidresin composition is used which hardens on exposure to air, or whenheated, and the mold is then dried or cured to give it the desiredmechanical strength.

In the present process, at least about 5% by volume of the material tobe infiltrated is comprised of a component which reacts with silicon.Silicon has an affinity for a substance with which it reacts and willmigrate toward such substance. Such a reactive component is required inorder for the silicon to infiltrate the porous body. Representative ofthe present component which reacts with silicon is elemental carbon anda metal such as, for example, molybdenum, titanium, chromium, tungsten,silver and aluminum. As used herein, the term "elemental carbon"includes all forms of elemental nondiamond carbon including graphite.

The component which reacts with silicon can be present in an amountranging from about 5% by volume to about 100% by volume of the materialto be infiltrated and the particular amount of such component dependslargely on the particular composite desired. Frequently, however, thecomponent which reacts with silicon is present in an amount of at leastabout 10% by volume, or at least about 20% by volume, or at least about50% by volume, of the material to be infiltrated.

Generally, that portion of the present material which is not reactivewith silicon is comprised of a ceramic material such as, for example,silicon carbide, silicon nitride, boron nitride and aluminum nitride.Diamond may be present as a nonreactive component depending largely onits particle size, and in a fine particle size it can react with siliconto form silicon carbide.

The present material to be infiltrated can be in a wide variety offorms. For example, it can be in the form of particles, filament, clothand mixtures thereof. The term filament herein includes fiber andwhisker.

The material to be infiltrated can be formed into a porous body by anumber of techniques. Preferably, the porous body is of the size andshape desired of the composite. For example, it can be extruded,injection molded, die-pressed, isostatically pressed or slip cast toproduce the porous body of desired size and shape. Any lubricants,binders, or similar materials used in shaping the material should haveno significant deleterious effect in the present process. Such materialsare of the type which evaporate on heating at temperatures below thepresent infiltration temperature, preferably below 500° C., leaving nodeleterious residue.

Alternately, if desired, the present material to be infiltrated can bepacked into the cavity of a mold of the present powder mixture to form apacked or constrained material, i.e. the present porous body.

The present porous body or packed material has an open porosity rangingfrom greater than about 10% by volume to about 90% by volume of thebody, and the particular amount of such open porosity depends largely onthe particular composite desired. Specifically, the porous body orpacked material can have an open porosity ranging from about 15% byvolume to about 80% by volume, or from about 30% by volume to about 60%by volume, of the body. By open porosity of the body, it is meant hereinpores or voids which are open to the surface of the body and therebymaking the interior surfaces accessible to the ambient atmosphere. Openporosity can be determined by standard metallographic techniques.Preferably, the present packed material or porous body to be infiltrateddoes not have any closed porosity or does not have any significantamount of closed porosity.

The pores in the porous body or packed material preferably should bedistributed uniformly or at least significantly uniformly to preventformation of excessively large pockets of elemental silicon which maylower the mechanical properties of the resulting infiltrated body orcomposite, which generally is a polycrystalline body, thereby limitingits applications. The pores can range in size, and generally can rangeup to about 2000 microns. For best results, the pores are submicron insize.

In carrying out the present process, a structure is formed comprised ofthe present mixture of silicon and hexagonal boron nitride in contactwith the porous body or packed material to be infiltrated. Such astructure can be formed by a number of techniques. In one embodiment,such a structure is comprised of the porous body with a deposit of thepressed powder mixture thereon. In another embodiment, the structure iscomprised of the material packed in a mold of the present powdermixture. The extent to which the present mixture is in contact with thesurface of the porous body depends largely on the particular compositedesired. Generally, the porous body or packed material is immersed in,or enveloped by, the present powder mixture leaving none of its surfaceexposed.

In a first specific embodiment of the present invention wherein a moldof the present mixture of silicon and hexagonal boron nitride powders isfirst formed, a predetermined quantity of the powder mixture is mixedwith a small amount of binder material and then placed in a boronnitride coated cavity machined in a graphite block. This damp mixture isthen pressed at a suitable pressure, usually about 100 psi, with a metalmaster the exact shape of the silicon infiltrated part desired. Aftercuring or drying, the metal master is removed and the resulting cavityof the powder mixture is filled with material that is to be infiltratedwith silicon. Additional powder mixture is placed on top of the filledcavity and pressed at a suitable pressure, preferably to about 100 psi,with a metal ram leaving none of the material exposed. The ram is thenremoved leaving a structure in the graphite cavity comprised of thepresent porous body enveloped by a mold of the present powder mixture.Preferably, hexagonal boron nitride powder is placed over the mixture.Porous carbon felt pads are then placed on top of the hexagonal boronnitride powder and held in place with a small block of graphite which issecured to the cavity-containing graphite with graphite screws orwrapped and tied with carbon fiber tow. The hexagonal boron nitridepowder placed over the mixed powder provides a barrier to preventexcessive silicon reaction with the carbon felt. The carbon felt holdsall of the powders in place during evacuation and heating. Also, thecarbon felt is very porous and allows gas to escape. The completegraphite assembly is placed in a vacuum furnace, evacuated to preferablyabout 0.1 torr, and heated to the required temperature for the timenecessary. When cool and restored to atmospheric pressure, the assemblycan be removed from the furnace, disassembled, and the siliconinfiltrated part removed from the powder mixture.

In a second specific embodiment of the present invention, a previouslyprepared preform of the material to be infiltrated, i.e. the presentporous body, is embedded or immersed, in a predetermined quantity of thepresent powder mixture contained in a boron nitride coated cavitymachined in a block of graphite. The powder mixture and preform arepressed under sufficient pressure, for example about 100 psi, withpreferably a metal ram. The ram is then removed leaving a structurecomprised of the porous body enveloped by the pressed powder mixturesupported in the graphite block, and from this point on, the procedureis the same as disclosed for the first specific embodiment.

The present structure or assembly is heated to infiltration temperaturein a nonoxidizing partial vacuum wherein the residual gases have nosignificantly deleterious effect on said structure or assembly and thepresent infiltration is carried out in such nonoxidizing partial vacuum.Preferably, such nonoxidizing partial vacuum is provided before heatingis initiated. The partial vacuum should be at least sufficient to removepockets of gas which may be trapped within the porous body which wouldblock the infiltrating fluid silicon. Generally, such a partial vacuumranges from about 0.01 torr to about 2 torr, and usually from about 0.01torr to about 1 torr to insure removal of entrapped gas in the bodybeing infiltrated.

Ordinarily and as a practical matter, the furnace used is a carbonfurnace, i.e. a furnace fabricated from elemental carbon. Such a furnaceacts as an oxygen getter for the atmosphere within the furnace reactingwith oxygen to produce CO or CO₂ and thereby provides a nonoxidizingatmosphere, i.e. the residual gases have no significantly deleteriouseffect on the infiltrating silicon. The present infiltration cannot becarried out in air because the liquid silicon would oxidize to formsolid silica before any significant infusion by silicon occurred. Insuch instance where a carbon furnace is not used, it is preferable tohave an oxygen getter present in the furnace chamber, such as elementalcarbon, in order to insure the maintenance of a nonoxidizing atmosphere.Alternatively, such nonoxidizing atmosphere, or atmosphere which has nosignificant deleterious effect on the structure within the furnace, canbe provided by a sufficiently high partial vacuum, i.e. about 10⁻² torrto 2 torr.

The present infiltration is carried out at a temperature at whichsilicon is fluid and which has no significant deleterious effect on thematerial being infiltrated. The present infiltration temperature rangesfrom a temperature at which the silicon becomes fluid to a temperatureat which there is no significant vaporization of the silicon.Preferably, the present infiltration temperature ranges from about 1375°C., or from about 1400° C., or from greater than about 1400° C. to about1600° C. By a temperature at which silicon is fluid it is meant herein atemperature at which the silicon is readily flowable. Specifically, whensilicon is at its melting temperature, it has a high viscosity, but asits temperature is raised, it becomes less viscous and at a temperatureabout ten degrees higher than its melting point, it becomes fluid. Themelting point of the silicon can vary depending largely on theparticular impurities which may be present. The temperature at which thesilicon is fluid is the temperature at which it will infuse orinfiltrate through the capillary-size passages, interstices or voids ofthe present packed material or porous body. With increase intemperature, the flowability of the fluid silicon increases resulting ina faster rate of infiltration and reaction.

The fluid silicon is highly mobile and highly reactive with elementalcarbon, i.e. it has an affinity for elemental carbon, wetting it andreacting with it to form silicon carbide. The fluid silicon also has anaffinity for several metals such as, for example, molybdenum, titanium,chromium, tungsten, silver and aluminum, reacting with them to form thesilicide.

Preferably, the composite produced by the present process has a porosityof less than about 10% by volume, more preferably less than about 5% byvolume, and most preferably less than about 1% by volume of thecomposite.

Preferably, in the present process, sufficient silicon is infiltratedinto the porous body, infusing or infiltrating through the voids orpores of the porous body by capillary action to react with all of theelemental carbon which may be present forming silicon carbide, or reactwith all the metal which may be present forming metal silicide, and alsoto fill any pores or voids which may remain producing an integral,strongly bonded infiltrated body or composite which has no detectableporosity, or no significant porosity.

The resulting infiltrated body or composite is cooled in an atmospherewhich has no significant deleterious effect on it, preferably it isfurnace cooled in the nonoxidizing partial vacuum to about roomtemperature, and the resulting composite is recovered.

The period of time required for infiltration by the silicon isdeterminable empirically and depends largely on the size of the of theporous body or packed material and extent of infiltration required, andfrequently it is completed within about 15 or 20 minutes.

Upon completion of the infiltration, a highly porous layer or deposit ofhexagonal boron nitride, or mostly hexagonal boron nitride, remainswhich is mechanically week and which is easily brushed or scraped offthe infiltrated body.

The present process not only eliminates the need for carbon wicks butalso eliminates the use of a reservoir for silicon. In addition, thepresent process makes possible the controlled infiltration of siliconinto the packed material or porous body in a uniform manner. Also, thepresent process does not leave any significant silicon nodules on thebody. In addition, the present process allows the production of aninfiltrated body directly in a wide range of sizes and shapes. Theresulting infiltrated body has a wide range of uses depending on itsparticular composition such as, for example, a tool insert or wearresistant part.

The invention is further illustrated by the following examples where,unless otherwise stated, the procedure was as follows:

Commercially pure 200 mesh (no greater than about 75 microns) siliconpowder was used.

Commercially pure 325 mesh (no greater than about 45 microns) hexagonalboron nitride powder was used.

Four parts of the silicon powder and one part of the hexagonal boronnitride powder were dry mixed in a conventional manner by stirring toproduce a substantially uniform mixture.

The "Epon 828" used is a resin formed from the reaction ofepichlorohydrin and Bisphenol A, which is a liquid at room temperatureand which has an epoxide equivalent of 185-192. Epon 828 decomposescompletely below 1300° C.

The curing agent used was diethylenetriamine, a liquid commonly calledDTA which cures Epon 828 thereby solidifying it.

EXAMPLE 1

A graphite block having a cylindrical cavity with an inner diameter ofabout 5/8 inch and about 5/8 inch deep was used. All of the innersurface of the cavity was sprayed with a slurry of hexagonal boronnitride which left a thin coating of the boride thereon. About 2 gramsof the present powder mixture of silicon and hexagonal boron nitride wasdampened with about a drop of Epon 828 and a thin continuous layer ofthe moist mixture was pressed under a pressure of about 100 psi onto aportion of the inner surface of the graphite cavity with a metal masterthe exact shape and dimensions of the disc desired. The metal master wasthen removed leaving a mold of the powder mixture with a disc-shapedcavity of 0.5 inch inner diameter and 0.05 inch deep. The graphite blockwas heated in an oven at 100° C. for about 10 minutes to cure the mold.0.16 gram of crushed carbon felt was placed in the disc-shaped cavity. Alayer of the powder mixture of silicon and hexagonal boron nitride wasdeposited on top of the crushed carbon felt leaving none of its surfaceexposed and pressed to about 100 psi with a metal ram forming acompacted carbon felt or porous body having an estimated open porosityof about 25% by volume of the body. The ram was then removed leaving astructure comprised of the porous body enveloped by a mold of the powdermixture. The mold had a porosity of about 35% by volume. A layer ofhexagonal boron nitride powder was deposited on top of the mold ofpowder mixture in the graphite cavity. Porous carbon felt pads wereplaced on top of the boron nitride powder and held in place with a smallblock of graphite which was placed on top of the pads and secured to thecavity-containing graphite block with graphite screws.

The complete graphite assembly was placed in a graphite vacuum furnacewhich was evacuted to about 0.1 torr and maintained at about 0.1 torrduring silicon infiltration and subsequent furnace-cooling to roomtemperature. The residual gases in the furnace were non-oxidizing.

The furnace was heated to about 1500° C. and maintained at suchtemperature for 10 minutes. The power was then cut off and the assemblywas furnace-cooled to room temperature.

A structure comprised of the silicon infiltrated disc in a porous moldwas recovered from the graphite cavity without sticking. The porous moldwas comprised mostly of hexagonal boron nitride and was easily scrapedoff the disc with a finger nail.

The infiltrated disc had no porosity, i.e. it was completelyinfiltrated, as determined by its density and by microscopicexamination.

The infiltrated disc, i.e. present composite, was comprised of siliconcarbide and silicon and would be useful as a wear resistant part.

EXAMPLE 2

This example was carried out in substantially the same manner as Example1 except that instead of packing the crushed carbon felt, a preform,i.e. porous body, of the crushed carbon felt was initially made, and thepowder mixture of silicon and hexagonal boron nitride was not formedinto a mold.

Specifically, at room temperature, DTA curing agent was mixed with 0.01gram Epon 828 resin in an amount of 10% by weight of the Epon 828, andthis mixture was then admixed with 0.16 gram of the crushed carbon felt.The resulting mixture was placed in the cylindrical cavity of a metalblock and pressed into the form of a disc with a metal plunger. Theresulting assembly was placed in a 100° C. oven for one hour to cure,i.e. harden, the Epon 828. The resulting disc was recovered withoutsticking. The disc was 0.5 inch in diameter, 0.05 inch thick and had anestimated open porosity of about 25% by volume of the disc.

The disc was immersed, i.e. enveloped, in the powder mixture of siliconand hexagonal boron nitride in the cavity of a graphite block precoatedwith hexagonal boron nitride and pressed with a metal ram at about 100psi compacting the powder mixture to a porosity of about 35% by volumeof the powder mixture. The remaining procedure to form the infiltrateddisc was the same as disclosed in Example 1.

The resulting silicon infiltrated disc was recovered from the graphitecavity without sticking. Some powder adhered to the disc but was easilyscraped off with a finger nail.

The infiltrated disc had no porosity, i.e. it was completelyinfiltrated, as determined by its density and by microscopicexamination.

The infiltrated disc, i.e. present composite, was comprised of siliconcarbide and silicon and would be useful as a wear resistant part.

What claimed is:
 1. A process for infiltrating a porous body withsilicon to form a composite which consists essentially of providing amaterial wherein at least about 5% by volume of said material comprisesa component which reacts with silicon, said component being selectedfrom the group consisting of elemental carbon, a metal and a mixturethereof, said material having a melting point higher than that of saidsilicon, forming a porous body of said material having an open porosityranging from greater than about 10% by volume to about 90% by volume ofsaid body, contacting said body with a mixture consisting essentially ofsilicon powder and hexagonal boron nitride powder wherein said siliconpowder ranges in amount from greater than about 10% by volume to lessthan about 90% by volume of said mixture, heating the resultingstructure in a nonoxidizing partial vacuum to an infiltrationtemperature at which said silicon is fluid but below the melting pointof said material, and infiltrating said fluid silicon into said porousbody forming said composite leaving a porous deposit of hexagonal boronnitride on said composite, said partial vacuum being at least sufficientto remove gas from said porous body which blocks said infiltrating fluidsilicon.
 2. The process according to claim 1 wherein said silicon powderhas a particle size ranging to about 150 microns and said hexagonalboron nitride has a particle size ranging to about 150 microns.
 3. Theprocess according to claim 1 wherein said silicon powder ranges inamount from about 60% by volume to about 80% by volume of said mixture.4. The process according to claim 1 wherein before said infiltration,said mixture in contact with said porous body has a porosity of lessthan about 50% by volume of said mixture.
 5. The process according toclaim 1 wherein said porous body is contacted with said mixture insupporting means and the resulting assembly is heated to saidinfiltration temperature, said supporting means having no significantdeleterious effect on said process.
 6. The process according to claim 1wherein said porous body is enveloped by said mixture in the cavity of agraphite piece and the resulting assembly is heated to said infiltrationtemperature.
 7. The process according to claim 1 wherein said porousbody has an open porosity ranging from about 15% by volume to about 80%by volume of said body and at least about 10% by volume of said materialcomprises a component which reacts with silicon.
 8. The processaccording to claim 1 wherein the remainder of said material is comprisedof a ceramic material selected from the group consisting essentially ofsilicon carbide, silicon nitride, boron nitride and aluminum nitride. 9.The process according to claim 1 wherein said metal is selected from thegroup consisting essentially of molybdenum, titanium, chromium,tungsten, silver and aluminum.
 10. A process for infiltrating a packedmaterial or porous body with silicon to form a composite which consistsessentially of forming a powder mixture consisting essentially ofsilicon powder and hexagonal boron nitride powder wherein said siliconpowder ranges in amount from greater than about 10% by volume to lessthan about 90% by volume of said mixture, shaping said mixture into amold having a cavity of the size and shape of said composite, providinga material wherein at least about 5% by volume of said materialcomprises a component which reacts with silicon, said component beingselected from the group consisting of elemental carbon, a metal and amixture thereof, said material being in the form of particles,filaments, cloth or a mixture thereof and having a melting point higherthan that of said silicon, packing said material into said cavityproducing a packed material or porous body therein having an openporosity ranging from greater than about 10% by volume to about 90% byvolume of said packed material or porous body, heating the resultingstructure in a nonoxidizing partial vacuum to an infiltratingtemperature at which said silicon is fluid but below the melting pointof said material, and infiltrating said fluid silicon into said packedmaterial or porous body forming said composite leaving a porous depositof hexagonal boron nitride on said composite, said partial vacuum beingat least sufficient to remove gas from said packed material or porousbody which blocks said infiltrating fluid silicon.
 11. The processaccording to claim 10 wherein said silicon powder has a particle sizeranging to about 150 microns and said hexagonal boron nitride has aparticle size ranging to about 150 microns.
 12. The process according toclaim 10 wherein said silicon powder ranges in amount from about 60% byvolume to about 80% by volume of said mixture.
 13. The process accordingto claim 10 wherein before said infiltration, said mixture in contactwith said porous body has a porosity of less than about 50% by volume ofsaid mixture.
 14. The process according to claim 10 wherein said mixtureis shaped into said mold in the cavity of supporting means and theresulting assembly is heated to said infiltration temperature, saidsupporting means having no significant deleterious effect on saidprocess.
 15. The process according to claim 10 wherein said mixture isshaped into said mold in the cavity of supporting means comprised ofgraphite.
 16. The process according to claim 10 wherein said packedmaterial or porous body has an open porosity ranging from about 15% byvolume to about 80% by volume of said body and at least about 10% byvolume of said material comprises a component which reacts with silicon.17. The process according to claim 10 wherein the remainder of saidmaterial is comprised of a ceramic material selected from the groupconsisting essentially of silicon carbide, silicon nitride, boronnitride and aluminum nitride.
 18. The process according to claim 10wherein said metal is selected from the group consisting essentially ofmolybdenum, titanium, chromium, tungsten, silver and aluminum.
 19. Theprocess according to claim 10 wherein said packed material or porousbody is enveloped by said powder mixture.
 20. A process for infiltratinga porous body with silicon to form a composite which consistsessentially of providing a material wherein at least about 20% by volumeof said material comprises a component which reacts with silicon, saidcomponent being selected from the group consisting of elemental carbon,a metal and a mixture thereof, said material having a melting pointhigher than that of said silicon, forming a porous body of said materialhaving an open porosity ranging from about 30% by volume to about 60% byvolume of said body, contacting said body with a mixture consistingessentially of silicon powder and hexagonal boron nitride powder whereinsaid silicon powder ranges in amount from about 50% by volume to about85% by volume of said mixture, heating the resulting structure in anonoxidizing partial vacuum to an infiltration temperature at which saidsilicon is fluid but below the melting point of said material, andinfiltrating said fluid silicon into said porous body forming saidcomposite leaving a porous deposit of hexagonal boron nitride on saidcomposite, said partial vacuum being at least sufficient to remove gasfrom said porous body which blocks said infiltrating fluid silicon.